Method for phase calibration in a frontend circuit of a near field communication device

ABSTRACT

A method for a phase calibration in a frontend circuit of a near field communication (NFC) tag device is disclosed. An active load modulation signal is generated with a preconfigured value of a phase difference with respect to a reference signal of an NFC signal generator device. An amplitude of a test signal present at an antenna of the NFC tag device is measured. The test signal results from overlaying of the reference signal with the active load modulation signal. The following steps are repeated: modifying the value of the phase difference, providing the active load modulation signal with the modified value of the phase difference, measuring an amplitude of the test signal and comparing the measured amplitude with the previously measured amplitude until the measured amplitude fulfills a predefined condition. The value of the phase difference corresponding to the previously measured amplitude is stored.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of International Patent ApplicationNo. PCT/EP2015/065352, filed on Jul. 6, 2015, which claims the benefitof U.S. Patent Provisional Application 62/040,983, filed Aug. 22, 2014,and also claims priority to European Patent Application No. 14182509.1,filed on Aug. 27, 2014, which claims the benefit of U.S. PatentProvisional Application No. 62/040,983, filed on Aug. 22, 2014, eachapplication is hereby incorporated herein by reference.

TECHNICAL FIELD

This application pertains to the near field communication (NFC) incontactless systems. In particular embodiments, this application isdirected to a method for a phase calibration in a frontend circuit of anNFC tag device, to a frontend circuit and to an NFC tag device.

BACKGROUND

NFC tag devices are employed in Radio Frequency Identification (RFID)systems that allow communication between an NFC tag device and acorresponding NFC reader device using inductive coupling. During thiscommunication the reader mainly sends commands to the tag. Subsequently,the tag answers by transmitting data, e.g., identification informationto the reader. Usually, tag and reader devices operate according toindustry standards. ISO/IEC 14443 is an example for such industrystandard which is also underlying the present application.

ISO/IEC 14443 defines contactless chip cards with an integrated NFC tagdevice in proximity coupling applications. Readers operating accordingto the standard use signals with a frequency of 13.56 MHz, calledcarrier frequency, to transfer data to a tag device. The tag device,which is also called a transponder, transmits data to the reader usingsignals with a frequency which is an integer divider of the carrierfrequency, e.g., 13.56 MHz/16 and is named the subcarrier frequency.According to ISO/IEC 14443, the transponder is powered by the field ofthe reader and applies a load modulation to the reader's signal whentransmitting data.

The operating range of RFID systems is limited by a coupling factorbetween an antenna of the reader and an antenna of the tag. The couplingfactor represents a measure for the strength of inductive couplingbetween reader and tag and basically is a function of distance and anglebetween reader and tag antennae, as well as, a function of mechanicalcharacteristics or geometry of the antennae. In order to increase theoperating range or to decrease the antenna size, concepts have beendeveloped which provide chip cards with a power source, e.g., a batterywhich enables these chip cards to actively generate a load modulatedsignal with the subcarrier frequency, thereby emulating the standardISO/IEC 14443 passive load modulation. This process is called activeload modulation.

The actively generated signal is synchronized to the signal of thereader. The active modulation allows using miniature antennas whilemaintaining transaction distance equal to or longer than the legacy,i.e., passive contactless cards. Having the option to use a miniatureantenna is advantageous in mobile phones or wearable devices where spaceis the most critical constraint. The cost of the antenna is alsoreduced. A long operating range is likewise important for a good userexperience and hence for the adoption of the contactless technology bythe mass market in applications such as mobile phones.

A tag or tag device that employs active load modulation is called anactive tag. The signal generated by the reader is also called areference signal, a carrier signal or a reader signal. The signalgenerated by the tag during active load modulation is also named activeload modulation signal.

SUMMARY

In one embodiment, a method for a phase calibration in a frontendcircuit of a near field communication (NFC) tag device is disclosed. Areference signal generated by an NFC signal generator device and a phasecalibration command are received. An active load modulation signal isgenerated with a preconfigured value of a phase difference with respectto the reference signal of the NFC signal generator device. An amplitudeof a test signal present at an antenna of the NFC tag device ismeasured. The test signal results from overlaying of the referencesignal with the active load modulation signal. The following steps arerepeated: modifying the value of the phase difference, providing theactive load modulation signal with the modified value of the phasedifference, measuring an amplitude of the test signal and comparing themeasured amplitude with the previously measured amplitude or with areference amplitude until the measured amplitude fulfills a predefinedcondition. The value of the phase difference corresponding to thepreviously measured amplitude is stored.

BRIEF DESCRIPTION OF THE DRAWINGS

The text below explains the proposed principle in detail using exemplaryembodiments with reference to the drawings. Components and circuitelements that are functionally identical or have the identical effectbear identical reference numbers. In so far as circuit parts orcomponents correspond to one another in function, a description of themwill not be repeated in each of the following figures.

FIG. 1 shows an NFC system;

FIG. 2 shows a test setup for a phase calibration;

FIG. 3 shows components of a tag device;

FIG. 4 shows a first exemplary embodiment of a method for a phasecalibration according to the proposed principle;

FIG. 5 shows an embodiment example of an NFC tag device with a frontendcircuit according to the proposed principle,

FIG. 6 shows a diagram with signals occurring at the antenna of the NFCtag of FIG. 5;

FIG. 7 shows a diagram of the amplitude of the test signal as a functionof a phase difference between reference signal and active loadmodulation signal;

FIG. 8 shows a diagram with the test signal at a test input of theproposed frontend circuit during the proposed method;

FIG. 9 shows an angle diagram of values of phase differences used inFIG. 4;

FIG. 10 shows a second exemplary embodiment of the method for a phasecalibration according to the proposed principle;

FIG. 11 shows a third exemplary embodiment of the method for a phasecalibration according to the proposed principle;

FIG. 12 shows a fourth exemplary embodiment of the method for a phasecalibration according to the proposed principle;

FIG. 13 shows a fifth exemplary embodiment of the method for a phasecalibration according to the proposed principle;

FIG. 14 shows a sixth exemplary embodiment of the method for a phasecalibration according to the proposed principle; and

FIG. 15 shows a seventh exemplary embodiment of the method for a phasecalibration according to the proposed principle.

The following reference symbols can be used in conjunction with thedrawings:

-   T, T′ Tag device-   R Reader device-   L Antenna-   MC Matching circuit-   FE, FE′ Front end circuit-   H, H′ Host component-   S1, S2, . . . , S9 Step-   S11, S21, S2 a, S31, S41, S4 a Step-   S51, S61, S71, S81, S91, S10 Step-   S8 a, S51 a, S8 b, S82, S92, S00 Step-   S01, S83, S83 a, S93, S84 Step-   Sr, St, Sf, Sc, Sd Signal-   V1, V2, V3, Vm1, Vm2, Vm3, Vm4 Amplitude-   Vmax, Vmin, Vref Amplitude-   P1, P2, Px Phase difference-   CLK Clock generator circuit-   CTL Control unit-   DRV Driver circuit-   MES Measurement circuit-   REC Receiver circuit-   Cb Coupling capacitance-   RFO1, RFO2 Signal output-   ANT1, ANT2 Test signal input-   t time-   VSS reference potential terminal-   C1, C2, C3, C4 condition.

DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS

A known NFC system that also uses active load modulation is depicted inFIG. 1. The system has an NFC tag device T′ and an NFC reader device R.The reader R emits a magnetic field by means of the carrier signal atthe carrier frequency via its antenna. The tag T′ responds with theactive load modulation signal using the subcarrier frequency. Ingeneral, the communication between the tag device T′ integrating theactive load modulation and the reader device R is half duplex. Thecontactless reader R sends a command and the tag device T′ in cardemulation mode answers to this command. Several commands, each of themfollowed by a card response, form a transaction. Transactions areperformed for payment, access control or transportation.

In legacy tag devices T′ passive load modulation is achieved byswitching a capacitive or a resistive load on its own antenna. In anactive load modulation which is compatible with ISO 14443, the tag T′generates a 13.56 MHz signal synchronized to the reader magnetic fieldto modulate the data. On the reader side, this load modulation signalhas exactly the same characteristics as the legacy passive loadmodulation signal. The strength of the load modulation is defined by theload modulation amplitude. The industry standards ISO 14443 or EMVCodefine limits for the parameters as well as the setup required for themeasurement. The benefit of the active load modulation is that the sameload modulation amplitude can be produced with an antenna which ishundred times smaller than the antenna of a passive tag.

In an active load modulation system, for a given coupling coefficient kbetween the reader R antenna and the tag T′ antenna, two parameters arecritical in generating the correct load modulation amplitude: a peak topeak voltage Vpp generated at the tag T′ antenna of a certain impedanceand a phase difference between the active modulation signal and thereader signal.

For a given coupling factor k and a given active load modulation signalamplitude at the card antenna, the maximum load modulation amplitudeseen at the reader antenna occurs when the phase difference between theactive load modulation signal and the reader signal is either 0° or 180°when measured at the tag T′ antenna. Any other phase difference valuewill result in a lower load modulation amplitude. This amplitude even iszero when the phase difference is either 90° or 270°. In that case, thedata is phase modulated, which is not compliant with EMVCo Contactlessstandard. The relationship between the load modulation amplitude and thephase difference is a cosine function.

The biggest challenge for a tag T′ employing active load modulationconsists in the ability to emit an active load modulation signal havinga constant and defined phase difference with respect to the readersignal. For a given coupling factor k between reader R and tag T′antennae, as well as for a given output power of the reader R and theactive load modulation signals, the amplitude modulation depth seen bythe reader's receiver depends exclusively on the phase differencegenerated by the tag T′. In order to achieve a desired phase difference,the tag T′ having active load modulation can generate any delay or phasedifference internally from 0° to 360° by use of a configurationregister. This phase difference compensates also for internal delaysexperienced by the reader signal and the generated load modulationsignal when travelling from the antenna across an external matchingcircuitry, internal signal processing circuitry and back to the antennathrough the external matching circuitry.

A problem arises from the fact that the delay described above has to beconfigured specifically for each active tag device produced because ofsilicon process variations, as well as tolerances within the externalmatching circuitry components and antenna. These variations may create a+/−75° variation of the phase difference for a given phase settingvalue. To ensure correct operation, the phase difference variationshould not exceed +/−30°. To fulfil these requirements, laborious testsare necessary. During these tests, available phase difference valueshave to be run through and the value that provides, for example, themaximum load modulation amplitude when measured at the reader side hasto be selected. This testing and calibration takes several tens ofseconds which is not acceptable for a production of high volume consumerdevices.

FIG. 2 shows the setup used for the above described test and phasecalibration process. The existing solution to calibrate the phase onactive tag devices is to use a phase calibration in the productionsequence. During this calibration step, each assembled device goesthrough a phase calibration procedure to determine the phase settingthat provides the best load modulation amplitude when measured onstandard measurement benches like, for example, on a Europay MasterCardVisa, EMV, Contactless Proximity Coupling Device.

For the phase calibration the NFC tag device T, called the device undertest (DUT) is placed near a proximity coupling device (PCD) which is anNFC device emulating a reader like a contactless point of sale terminalfor instance. The reader signal is generated by a signal generator,amplified by a power amplifier and then fed to the PCD. The test systemcauses the DUT to use a first phase setting. The PCD then generates areader command and the DUT generates a card answer by means of an activeload modulation signal using this first phase setting. The amplitude ofthe resulting load modulation signal is measured at the PCD's antenna bymeans of an oscilloscope in a load modulation amplitude (LMA)measurement.

For this purpose, the PCD is connected to the oscilloscope. The DUT andmeasurement equipment are controlled by the test system. The value ofthe measured amplitude is stored. The test system causes the DUT to usethe next phase setting for generating the load modulated signal and theresulting amplitude is again measured on the antenna of the readerdevice. The test system proceeds by repeating these steps and therebysweeps through all phase configuration settings stored in the DUT. Bythis, the phase setting with the maximum LMA measured at the readerantenna is determined and configured in the DUT.

However, the described phase calibration process approximately takesseveral tens of seconds in the production which is too long for a highvolume production.

FIG. 3 shows the main building blocks of the DUT. The DUT has an antennaL, a matching circuit MC′, a frontend circuit, FE′ and a host componentH′. The antenna L is coupled via the matching circuit MC′ to thefrontend circuit FE′. The host component H′ is coupled by a digitalinterface to the frontend circuit FE′. The host component H′ controlsthe frontend circuit FE′. It interprets the commands received from areader device and provides the data which is to be returned to thereader in answering the commands. The frontend circuit FE′ realizes thehigh frequency functionality necessary in generating and receivingsignals by means of the antenna L and the matching circuit MC′.

The present application starts out from a test setup as described inFIG. 2. Although the currently used equipment is based on an automationof sweeping through the phase settings in a tag device, the test timesthat can be achieved are too long. According to embodiments of theinvention, a method for a phase calibration, a corresponding frontendcircuit and an NFC tag device can achieve shorter test times.

The definitions as described above also apply to the description of thebelow embodiments unless stated otherwise.

In one embodiment, a method for a phase calibration in a frontendcircuit of a near field communication (NFC) tag device is disclosed. Areference signal generated by an NFC signal generator device and a phasecalibration command are received. An active load modulation signal isgenerated with a preconfigured value of a phase difference with respectto the reference signal of the NFC signal generator device. An amplitudeof a test signal present at an antenna of the NFC tag device ismeasured. The test signal results from overlaying of the referencesignal with the active load modulation signal. The following steps arerepeated: modifying the value of the phase difference, providing theactive load modulation signal with the modified value of the phasedifference, measuring an amplitude of the test signal and comparing themeasured amplitude with the previously measured amplitude or with areference amplitude until the measured amplitude fulfills a predefinedcondition. The value of the phase difference corresponding to thepreviously measured amplitude is stored.

All the steps of the proposed method are completed within the frontendcircuit of the NFC tag device. After receiving the phase calibrationcommand, the frontend circuit itself goes through the different valuesor settings of the phase difference and measures the amplitude of theresulting signal at its own antenna until the amplitude which matchesthe predefined condition is detected. The frontend circuit is configuredto the value of the phase difference which corresponds to the amplitude.Thereby, the phase of the active load modulation signal is configuredwith respect to the phase of the reference signal of the NFC signalgenerator device.

As the method is processed within the frontend circuit, the time whichpreviously had to be spent on sending commands for activating adifferent setting, sending reader commands, reading the card answer,measuring the amplitude on the reader device and generating new commandsfor a different setting is saved. The process can be completed inapproximately one millisecond.

According to embodiments of the invention, due to the auto-calibrationof the phase of the active load modulation signal, the duration of thecalibration process is reduced.

The test signal generator device can be an NFC reader device, or a testdevice like the standard EMVCo tester which generates the referencesignal which represents the carrier signal having the defined carrierfrequency as of ISO 14443, for example.

The phase difference is a value indicated in degrees which defines thedifference in phase between two signals, here between the referencesignal and the active load modulation signal. The frontend circuitimplements various phase difference values which differ from each other.The proposed method performs measurements of the amplitude at the tagdevice antenna using different values for the phase difference until themeasured amplitude fulfills the predefined condition.

In an alternative embodiment the step receiving a phase calibrationcommand is performed prior to the step receiving a reference signal.

In a development the predefined condition comprises at least one of

-   -   the measured amplitude is smaller than the previously measured        amplitude,    -   the measured amplitude is bigger than the previously measured        amplitude,    -   the measured amplitude is smaller than the reference amplitude,        or    -   the measured amplitude is bigger than the reference amplitude.

In the first case, in which the method is performed as long as themeasured amplitude is smaller than the previously measured amplitude, amaximum of the amplitude of the test signal and the corresponding valueof the phase difference are detected.

In the second case, in which the method is performed as long as themeasured amplitude is bigger than the previously measured amplitude, aminimum of the amplitude of the test signal and the corresponding valueof the phase difference are detected.

In the third case, in which the method is performed until the measuredamplitude is smaller than the reference amplitude, the amplitude of thetest signal and the corresponding value of the phase difference aredetermined which match the reference amplitude when approaching thereference amplitude from a higher amplitude value.

In the fourth case, in which the method is performed until the measuredamplitude is bigger than the reference amplitude, the amplitude of thetest signal and the corresponding value of the phase difference aredetermined which match the reference amplitude when approaching thereference amplitude from a lower amplitude value.

In a development the method further comprises: measuring an amplitude ofthe test signal during non-emission of the active load modulationsignal, and recording this amplitude as the reference amplitude.

In order to determine the reference amplitude, the amplitude of the testsignal is measured at a point in time at which the active loadmodulation signal is not being provided. The measured amplitude thenrepresents the amplitude of the reference signal, because the referencesignal is provided continuously during the phase calibration. Anexemplary point in time for determining the reference amplitude is rightafter the receiving the phase calibration command step.

According to one embodiment, the active load modulation signal isgenerated in function of an internal carrier signal and a data signal.

According to a first possibility, the active load modulation signal isgenerated as a burst in function of the internal carrier signal and thedata signal during a short period of time. According to anotherpossibility the active load modulation signal is a function of an activeload modulation of the internal carrier signal with the data signal.

Consequently, the active load modulation signal is emitted with afrequency synchronized to the frequency of the reference signal and witha configured phase difference to the reference signal during a shortperiod of time. This period preferably is long enough to perform themeasurement of the amplitude of the resulting test signal.

In case of the ISO 14443 standard, the data signal comprises the data tobe transmitted which are coded as defined in the standard and modulatedwith a signal having the subcarrier frequency. Embodiment methods of thepresent disclosure do not require any specific data or data pattern.Instead, the signal having the subcarrier frequency is used for themodulation with the internal carrier signal to generate the active loadmodulation signal.

In the case of other standards, for example, Felica or ISO 15693, thedata signal comprises the data to be transmitted which are coded in aspecified manner. However, the standards do not employ the modulationusing the subcarrier frequency.

In case of the active load modulation signal being generated in the formof a burst, the data signal supplied by the control unit comprises theburst which means at least information regarding a start and a stoppoint in time. Consequently, the active load modulation signal isprovided for a period of time defined by the start and stop point intime.

The tag device, which implements embodiment methods of the presentdisclosure, uses active load modulation as described above. It may alsoimplement state of the art passive load modulation.

In a development the modifying of the value of the phase differencerealizes an increase or a decrease of the phase difference of the activeload modulation signal with respect to the reference signal.

In order to find the amplitude of the test signal occurring at anantenna of the tag device the amplitude matching the predefinedcondition, and starting out from the preconfigured value of the phasedifference, the phase difference can either be increased or decreasedwhen switching to the next value of the phase difference and measuringagain the amplitude of the test signal. The preconfigured value can alsobe called the default value.

In a further development the values of the phase difference areretrieved from a memory in the frontend circuit or in the host componentof the NFC tag device.

Various values of the phase difference which configure the phase of theactive load modulation signal to a fixed phase difference with respectto the reference signal are stored within the frontend circuit. Thesevalues are used one after the other until the amplitude maximum, theamplitude minimum or the amplitude matching the reference amplitude havebeen detected. The memory can be volatile or non-volatile.

In one embodiment the phase calibration command is generated by a hostcomponent of the NFC tag device.

The command which triggers the auto phase calibration according toembodiment methods of the present disclosure is generated and emitted bythe host component of the tag device as soon as the host componentdetects the reference signal emitted by the signal generator device.

In another embodiment the test signal comprises voltage values which areproportional to a voltage at the antenna of the NFC tag device.

Consequently, the amplitude of the test signal is determined bymeasuring the voltage level at the antenna of the NFC tag device.

In a development the internal carrier signal is generated using thereference signal. A frequency of the internal carrier signal is adaptedto a frequency of the reference signal.

A frequency of the reference signal is called carrier frequency andamounts to, for instance, 13.56 MHz, when using the ISO 14443 standard.The subcarrier frequency in this case is determined by dividing thecarrier frequency by 16 and amounts to 848 KHz.

In a development a period during which the active load modulation signalis not emitted is present between two consecutive amplitude measurementsof the test signal.

During this period the internal carrier signal is resynchronized to thereference signal. This can be achieved by means of a phase-locked loop,for instance.

In one embodiment a frontend circuit for a near field communication,NFC, tag comprises a test signal input for receiving a test signal whichis proportional to a signal occurring at an antenna which can beconnected to the frontend circuit and a signal output for providing anactive load modulation signal. The test signal input is further preparedfor receiving a reference signal generated by an NFC signal generatordevice. The frontend circuit further has a control unit, a receivercircuit, a measurement circuit, a clock generator circuit and a drivercircuit. The control unit is prepared to run and control embodimentmethods of the present disclosure as described above and to provide avalue of a phase difference and a data signal.

The receiver circuit is coupled to the test signal input and is preparedto perform envelope detection on the reference signal and on the testsignal. The measurement circuit is coupled to the receiver circuit andto the control unit. The measurement circuit is configured to provide anamplitude of the test signal. The clock generator circuit is coupled tothe receiver circuit and to the control unit and is adapted to generatean internal carrier signal using the value of the phase difference. Thedriver circuit is coupled to the clock generator circuit and to thesignal output. The driver circuit is configured to provide the activeload modulation signal as a function of the internal carrier signal withthe data signal.

As soon as the reference signal is detected by the test signal input andthe receiver circuit, the control unit initiates the generation of theactive load modulation signal with the help of the clock generatorcircuit and the driver circuit. The active load modulation signal isprovided at the signal output and emitted by means of the connectableantenna. An overlay of the active load modulation signal with thereference signal occurs at the antenna and is tapped as test signal atthe test signal input. The measurement circuit provides the amplitude ofthe test signal. The control unit subsequently configures the phasedifference by choosing the next value of the phase difference andprovides it to the clock generator circuit. Consequently, the activeload modulation signal is provided using the next value of the phasedifference and the amplitude of the test signal is measured once again.As described above, the process is repeated until the amplitude of atest signal which complies with the predefined condition is found. Thecorresponding value of the phase difference is used as a calibratedvalue for the frontend circuit in operation. Alternatively, a valuederived from the value may be used in operation.

As the phase calibration takes place completely within the frontendcircuit, the calibrated value of the phase difference for the NFC tagdevice in production can be read out shortly after starting the method,e.g. in one millisecond, which greatly reduces the time for calibrationand testing.

In a development the measurement circuit comprises an analog-to-digitalconverter circuit.

The analog-to-digital converter determines the amplitude of the testsignal based on the envelope provided by the receiver circuit.

In one embodiment an NFC tag device has a frontend circuit as describedabove, an antenna, a host component which is coupled to the frontendcircuit by means of a digital interface and a matching circuit which isconnected between the frontend circuit and the antenna.

The value of the phase difference which produces the amplitude of thetest signal that fulfills the predefined condition is stored either inthe frontend circuit or in the host component in a non-volatile form.

In a development the matching circuit of the tag device comprises aseries capacitor which is coupled between the antenna and the testsignal input of the frontend circuit.

The series capacitor may even be directly coupled between the antennaand the test signal input. The antenna is formed by at least one coil.

The series capacitor allows dividing a voltage at the antenna by a fixedvalue depending on the capacitance value.

In another development the series capacitor forms a capacitive dividerwith a capacitance of the test signal input.

As a consequence of the series capacitor, an amplitude of the testsignal is proportional to the voltage at the antenna of the tag device.

FIG. 4 shows a first exemplary embodiment of a method for a phasecalibration according to the proposed principle.

In step S1, a frontend circuit receives a reference signal which isgenerated by an NFC test signal generator device.

In step S2, the frontend circuit receives a phase calibration command.Therein, the phase calibration command is provided e.g. by a hostcomponent of an NFC tag device whose frontend circuit is being phasecalibrated.

In step S3, the frontend circuit generates an active load modulationsignal with a preconfigured phase difference with respect to thereceived reference signal. Different values of the phase differencesused one after the other in the described method are stored beforehandin a memory, e.g. registers in the frontend circuit. It is also definedbeforehand which of the stored values is used in the first place as thepreconfigured or default value of the phase difference.

In step S4, an amplitude of a test signal present at an antenna which isconnectable to the frontend circuit is measured. The amplitude may bedetermined as a voltage value.

In step S5, the value of the phase difference is modified. This meansthat the next value of the phase difference is retrieved from thestorage, e.g. register. This leads to an increase or a decrease of theactual phase difference between the reference signal and the active loadmodulation signal.

In step S6, the frontend circuit provides the active load modulationsignal with this modified phase difference value.

During step S7, another measurement of the amplitude of the test signalis conducted.

In step S8, this measured amplitude is compared to the previouslymeasured amplitude or to a reference amplitude. As long as the newlymeasured amplitude does not fulfil the predefined condition, steps S5,S6, S7, and S8 are repeated.

Otherwise, i.e. if the newly measured amplitude fulfills the predefinedcondition, the desired amplitude has been found in the previousmeasurement. Consequently, the previous value of the phase difference,which corresponds to the previously measured amplitude, represents thevalue of the phase difference to which the frontend circuit is to becalibrated.

Therefore, in step S9, the phase difference corresponding to theprevious amplitude, which has been named previous value of the phasedifference, is stored. The phase difference value can either be storeddirectly in the frontend circuit or can be stored in a host component ofthe NFC tag device under test after reading out the value of the phasedifference by the host component.

Optionally, a value of the phase difference derived from the valuedetermined by means of the method may be stored.

Due to the described embedded auto-phase calibration in the frontendcircuit, the desired phase setting which realizes a defined phasedifference between the active load modulation signal and the referencesignal can be determined in approximately less than 1 ms after receptionof the reference signal. This represents a significant reduction in timefor a calibration of frontend circuits.

FIG. 5 shows an embodiment example of an NFC tag device with a frontendcircuit according to the proposed principle. The tag device T comprisesthe frontend circuit FE, a matching circuit MC, a host component H, andan antenna L. The antenna L is connected via the matching circuit MC torespective inputs and outputs of the frontend circuit FE. The antenna Lis prepared to emit and receive NFC signals as required in therespective standards and as known to a person skilled in the art. Thehost component H is coupled to the frontend circuit FE by means of asuitable interface, for example, by a digital interface.

The frontend circuit FE uses a power supply of the host component H.Alternatively, the frontend circuit FE may be powered by a superordinatedevice which integrates the tag T. This superordinate device may be amobile phone, for instance.

The matching circuit MC has different inductors, capacitances andresistors to connect pins of the antenna L to the frontend circuit FE asis well known to someone skilled in the art. Furthermore, each antennapin is coupled via a series capacitance Cb to a test input ANT1, ANT2 ofthe frontend circuit FE. Each series capacitance Cb is configured toform a capacitive divider with a capacitance of the test input ANT1,ANT2 in order to deliver a test signal St or a reference signal Sr tothe test inputs ANT1, ANT2, which is reflecting the voltage at theantenna L.

The frontend circuit FE comprises a signal output RFO1, RFO2 forproviding an active load modulation signal Sf and the test signal inputANT1, ANT2 for receiving the reference signal Sr and the test signal St.As differential signals are used in this exemplary embodiment, inputsand outputs of the frontend circuit FE are prepared correspondingly. Inan alternative implementation, single-ended signals may be employed.

Furthermore, frontend circuit FE comprises a control unit CTL which isprepared to run and control the method described above. The control unitCTL provides several values of the phase differences P1, P2, PX and adata signal Sd. Moreover, the frontend circuit FE comprises a receivercircuit REC, a measurement circuit MES, a clock generation circuit CLKand a driver circuit DRV. The receiver circuit REC is coupled to thetest signal input ANT1, ANT2 and is prepared to perform an envelopedetection of the reference signal Sr or the test signal St. The receivercircuit REC is furthermore coupled to the control unit CTL and to areference potential terminal VSS. The measurement circuit MES is coupledto the receiver circuit REC and to the control unit CTL. The measurementcircuit MES is configured to provide an amplitude as will be describedin more detail with reference to FIG. 8 of the test signal St. The clockgenerator circuit CLK is coupled to the receiver circuit REC and to thecontrol unit CTL. The clock generator circuit CLK is adapted to generatean internal carrier signal Sc using the phase difference P1, P2, PX. Thedriver circuit DRV is coupled to the clock generator circuit CLK and tothe signal output RFO1, RFO2. The driver circuit DRV is furthermoreconnected to the reference potential terminal VSS. The driver circuitDRV is configured to modulate the internal carrier signal Sc with thedata signal Sd in order to provide the active load modulation signal Sfwhich is emitted by the antenna L.

The depicted tag T can be used in the state-of-the-art test setup asdescribed in FIG. 2.

The measurement circuit MES comprises amongst others ananalog-to-digital converter circuit which completes determination of arespective amplitude of the test signal St when the method is executed.For this, an envelope e of the test signal St is detected by thereceiver circuit REC and its amplitude is measured by theanalog-to-digital converter.

The reference signal Sr is received at the test signal inputs ANT1,ANT2. The clock generator circuit CLK generates the internal carriersignal Sc using a clock extracted from the reference signal Sr by thereceiver circuit REC. Consequently, a frequency of the internal carriersignal Sc is adapted to a frequency of the reference signal Sr. Thedriver circuit DRV modulates the internal carrier signal Sc with thedata signal Sd, thereby providing the active load modulation signal Sf.Alternatively, the driver circuit DRV provides the active loadmodulation signal Sf as a burst in function of the internal carriersignal Sc and the data signal Sd during a short period of time.Consequently, the active load modulation signal Sf is synchronized infrequency to the reference signal Sr and emitted at the signal outputsRFO1, RFO2. In order to keep the same order of amplitude for bothsignals at the antenna L, a supply voltage of the driver circuit DRV canbe configured to its lower value or the driver circuit's DRV resistanceis increased. The phase difference between the signals observed at theantenna L is configured to the different values P1, P2, PX which areretrieved from internal registers, for example. The depth and polarityof the amplitude modulation observed in the overlaid signal at theantenna L, i.e. the test signal St, depends on the phase of the activeload modulation signal Sf as described above.

In order to determine the reference amplitude, an amplitude of thereference signal Sr is measured by the measurement circuit MES at thetest signal input ANT1, ANT2 during a period when the active loadmodulation signal Sf is not emitted.

The short period of time during which the active load modulation signalSf is provided as a burst amounts to, for example, several periods toseveral tens periods of the internal carrier signal Sc.

In an alternative implementation, embodiment methods of the presentdisclosure may be executed by the host component H of the tag device T.The host component H can be realized as an NFC controller or a secureelement as known in the art.

FIG. 6 shows a diagram with signals occurring at the antenna of the NFCtag of FIG. 5. The abscissa represents time t, the ordinate showsvoltage values of reference signal Sr and test signal St. Amplitudevalues V1, V2, and V3 are depicted which each represent a peak-to-peakvoltage or a peak voltage of the envelope of the signal detected at thetest input of the frontend circuit as of FIG. 5.

The frontend circuit provides the active load modulation signal Sfaccording to the standard used. This results in periods with only thereference signal Sr being present at the test input alternating withperiods during which the test signal St reflects the overlay of theactive load modulation signal Sf with the reference signal Sr.

In the exemplary case of the standard ISO 14443 which employs asubcarrier, the frontend circuit provides the active load modulationsignal Sf for instance during half of a period of the subcarrier. Forinstance, the active load modulation signal is turned on during thefirst half of this period and turned off during the second half of thisperiod. Consequently, in an exemplary implementation, the measuring ofan amplitude of the test signal St is performed during one half of asubcarrier period during which the active load modulation signal Sf isprovided. In other words, in the exemplary case of ISO 14443, since thesubcarrier is 847 kHz, the active load modulation signal Sf is emittedduring one half of the subcarrier frequency period which corresponds toeight periods of the carrier frequency. There is no emission during theother half during which the receiver circuit is resynchronized to thereference signal Sr.

Vref is the amplitude of the test signal St which occurs during theperiod in which the active load modulation signal is not emitted. Itcorresponds to the amplitude of the reference signal Sr which is sentout by the test signal generator device. Following the abovedescription, amplitude Vref represents the reference amplitude.

Vmax is the amplitude of the test signal St during the period whenactive load modulation is performed and the active load modulationsignal is switched on and provided at the signal output of the frontendcircuit. The amplitude Vmax therefore is the amplitude resulting fromoverlaying the reference signal Sr with the active load modulationsignal Sf which results in the test signal St. The overlaying of signalscan also be designated a superimposition. In the depicted case, theamplitude Vmax represents the maximum amplitude of the test signal Stwhich corresponds to a phase difference between the overlaid signals of0°. A difference between the maximum amplitude Vmax and the referenceamplitude Vref is positive and maximum. Amplitude Vmax as the maximumamplitude fulfills one predefined condition of embodiment methods of thepresent disclosure.

A minimum amplitude Vmin results from an overlay of the reference signalSr with the active load modulation signal Sf using another phasedifference which in this example amounts to 180°. A difference betweenthe minimum amplitude Vmin and the reference amplitude Vref is negativewith an absolute value also representing a maximum. Amplitude Vminrepresents the minimum amplitude of the test signal St and also fulfillsone predefined condition of embodiment methods of the presentdisclosure.

When the phase difference is between 0° and 90° or between 270° and360°, the amplitude of the test signal St lies between Vref and Vmax.When the phase difference amounts to exactly 90° or exactly 270°, theamplitude of the resulting test signal St is equal to the referenceamplitude Vref. When the phase difference lies between 90° and 270°, theamplitude of the resulting test signal St lies between Vmin and Vref.

FIG. 7 shows a diagram of the amplitude of the test signal as a functionof a phase difference between the reference signal and the active loadmodulation signal occurring at the antenna of the proposed NFC tagdevice. The abscissa depicts values for the phase difference in degrees,whereas the ordinate represents amplitudes of the test signal St duringemission of the active load modulation signal.

As described above, a phase difference of zero degrees between theemitted active load modulation signal and the reference signal resultsin a maximum amplitude Vmax of the test signal St. This represents afirst predefined condition C1 according to embodiment methods of thepresent disclosure.

A phase difference of 180° between the active load modulation signal andthe reference signal leads to a minimum amplitude Vmin of the testsignal St. This represents a second predefined condition C2 according toembodiment methods of the present disclosure.

A phase difference of 90° results in the test signal St taking on areference amplitude Vref which corresponds to the amplitude of thereference signal. This represents a third predefined condition C3 of themeasured amplitude of the test signal St according to a method of thepresent disclosure.

A phase difference of 270° also results in the test signal St having thereference amplitude Vref. This represents a fourth predefined conditionC4 of the measured amplitude according to embodiment methods of thepresent disclosure.

It can be seen from FIG. 7 that the relationship between the phasedifference at the antenna and the amplitude of the test signal St is acosine function. Embodiment methods of the present disclosure enabledetermining which of the stored values of phase difference which is usedfor generating the active load modulation signal leads to which realphase difference occurring at the antenna of the tag device by detectingone of the four predefined conditions C1, C2, C3 or C4. Based on thesefindings, the frontend circuit can be calibrated to the desired value ofthe phase difference which is to be used in generation of the activeload modulation signal.

FIG. 8 shows a diagram of the test signal occurring at the test input ofthe proposed frontend circuit during the proposed method. Like in FIG.6, voltage values of reference signal Sr and test signal St are shownwith reference to time t. The amplitude of the reference signal Sr isdesignated Vref. The amplitudes of the test signal St measured at thetest signal input of the frontend circuit each time using a differentvalue of the phase difference are designated Vm1, Vm2, Vm3, and Vm4. Ineach case, an envelope e of the test signal St is detected.

It is predefined in this case that the method determines the phasesetting of the maximum amplitude of the test signal St.

As can be seen, the first value of the phase difference used inembodiment methods of the present disclosure as the preconfigured valueof the phase difference results in amplitude Vm1 of the test signal Stpresent during the first half of the subcarrier period. The frontendcircuit switches to the next value of the phase difference in phasesetting 2 and again determines the amplitude of the test signal Stresulting therefrom, which amounts to Vm2. Comparison of Vm2 to Vm1reveals that the actual value of the amplitude Vm2 is bigger than thepreviously measured value Vm1.

Consequently, the frontend circuit switches to the next value of thephase difference in phase setting 3, provides the active load modulationsignal using this phase difference and measures the amplitude of theresulting test signal St which amounts to Vm3. Amplitude Vm3 is comparedto the previously measured amplitude Vm2. Because Vm3 is bigger thanVm2, the frontend circuit switches to the next value of the phasedifference and repeats the provisioning of the active load modulationsignal and the measurement of the test signal St. The resultingamplitude Vm4 this time is determined to be smaller than the previouslymeasured amplitude Vm3. Consequently, amplitude Vm3 is determined to bethe maximum amplitude of the test signal St. It fulfills the predefinedcondition. The method returns the corresponding value of the phasedifference, i.e., phase setting 3.

By this, the possible values of the phase difference which have beenstored beforehand, e.g., in configuration registers, are tested in turnat each subcarrier period. At each period, the resulting amplitude VmXis measured. The phase difference which provides the largest VmXamplitude is stored as the optimized value.

As during each subcarrier period, one phase difference value can betested, the maximum length of the calibration process is the duration ofthe subcarrier period multiplied by the number of stored values of phasedifferences. The duration of the subcarrier period amounts toapproximately 1.2 microseconds. Consequently, time that has to be spenton calibration during production is greatly reduced when compared to thestate of the aft.

FIG. 9 depicts an angle diagram of values of the phase difference usedin the method of FIG. 4. The initial value of the phase difference usedin the method as preconfigured phase difference is P1. The phasedifference amounts to approximately 210°. The method continues usingdifferent values of the phase difference and measuring the amplitudes asdescribed and thereby moves in the direction of the arrow. Finally,phase difference value Px is determined, which corresponds to themaximum amplitude measured at the test signal input of the frontendcircuit. Px represents a phase difference of zero degrees.

FIG. 10 shows a second exemplary embodiment method of the presentdisclosure. In this embodiment the method as described for example inFIG. 4 is configured to detect the amplitude of the test signal thatsatisfies the first condition C1 as described in FIG. 7. That means thatthe maximum amplitude of the test signal is searched for.

In step S11, the frontend circuit receives the reference signalgenerated by the NFC test signal generator device.

In step S21, the frontend circuit receives the phase calibration commandprovided by the host component of the NFC tag device whose frontendcircuit is being phase calibrated.

In step S2 a, an initial phase setting is configured by choosing aninitial value for the phase difference for the active load modulationsignal from the memory.

In step S31, the frontend circuit generates the active load modulationsignal using the initial phase difference value. The initially usedvalue is the default value.

In step S41, an amplitude of the test signal occurring during emissionof the active load modulation signal is measured.

This amplitude is recorded as max_amplitude in step S4 a. Concurrentlyan index of the used phase setting is recorded as max_index.

In the following, the term index refers to a pointer which identifies aphase setting corresponding to a value of the phase difference.

In step S51, the value of the phase difference is configured to the nextvalue with respect to the storage place in the memory. This leads to anincrease of the phase difference.

In step S61, the active load modulation signal is generated using thenext phase setting.

In step S71, another measurement of the amplitude of the test signal isconducted.

In step S81, this newly measured amplitude is compared to themax_amplitude. If the measured amplitude is bigger than themax_amplitude, the measured amplitude is recorded as max_amplitude andthe index of the corresponding phase setting is recorded as max_index instep S8 a. Subsequently, the steps S51, S61, S71 and S81 are repeated.

As soon as it is determined in step S81 that the measured amplitude issmaller than the max_amplitude, the described loop function isterminated.

In step S10 it is checked whether all possible phase difference valueswhich are stored in the tag device have been tested. The loop isre-entered at the step S51 if not all of the possible values for thephase difference have been tested. As soon as all the phase settingshave been tested, step S91 is performed.

The max_index is stored in a non-volatile memory in step S91. Therein,the max_index indicates the value of the phase difference out of thelist of pre-stored values of phase differences to be used in generatingthe active load modulation signal which leads to a maximum amplitude ofa test signal and consequently realizes an actual phase difference ofabout 0° with respect to the reference signal.

Thereby, the max_index can be stored in non-volatile memory of thecontrol unit of the frontend circuit, or the host component of the tagdevice reads the max_index by means of the digital interface and storesit in its own non-volatile memory.

FIG. 11 shows a third embodiment of a method of the present disclosure.In this embodiment, the method is also configured to find the maximumamplitude of the test signal defined as first condition C1 in FIG. 7.This means it is detected which value of the phase difference or whichphase setting has to be configured in generation of the active loadmodulation signal that achieves a phase difference of zero degrees inrelation to the reference signal emitted by a signal generator device ora reader device.

The method of FIG. 11 matches the embodiment described in FIG. 10regarding steps S11, S21, S2 a, S31, S41, S4 a, S51, S61, S71, S81, S8 aand S91.

The difference to the embodiment of FIG. 10 consists in an additionalstep S51 a which is completed if it is determined in step S81 that themeasured amplitude is not bigger than the max_amplitude. In step S51 a,the phase difference is modified by configuring a predecessor phasesetting with respect to the storage place of the values of the phasedifference in memory. This leads to a decrease of the actual phasedifference used in generating the active load modulation signal.

Subsequently, the active load modulation signal is generated in step S61using the newly configured phase setting and the amplitude of the testsignal is measured at the antenna during its emission in step S71. Onceagain, the measured amplitude is compared to the max_amplitude. If themeasured amplitude is bigger, then this amplitude is recorded asmax_amplitude and the corresponding phase setting index is recorded asmax_index. Then steps S51 a, S61, S71 and S81 are repeated until themeasured amplitude is smaller than the max_amplitude. In that case, stepS91 is completed and the max_index is stored in non-volatile memory asdescribed above.

This embodiment achieves a further acceleration of the determination ofthe phase difference value which leads to the maximum amplitude of thetest signal.

In an exemplary implementation each of the various values of the phasedifference stored in the frontend circuit is addressed via an index i.This index is set to zero at the beginning of the method in order topoint to the preconfigured value of the phase difference. Thecorresponding amplitude of the test signal is measured. While themeasured amplitude corresponding to an incremented value of the index iis bigger than the previously measured amplitude, measurement of theamplitude is continued. The index i is decremented. While the measuredamplitude corresponding to a decremented value of the index i is biggerthan the previously measured amplitude, measurement of the amplitude iscontinued. The index i is returned to the host component.

FIG. 12 shows a fourth embodiment method of the present disclosure. Inthis embodiment the method is configured to determine the phasedifference value which leads to the minimum amplitude of the test signaland thereby realizes a 180° phase shift between the active loadmodulation signal and the reference signal. This implements condition C2as of FIG. 7.

Steps S11 to S41 correspond to steps S11 to S41 as described in FIGS. 10and 11.

In contrast to FIGS. 11 and 10, in the present embodiment in step S4 bthe amplitude measured in step S41 is recorded as min_amplitude and anindex of the corresponding value of the phase difference is recorded asmin_index.

The method proceeds as described by modifying the phase difference byconfiguring the next phase setting and thereby increasing the phasedifference in step S51, generating the active load modulation signal instep S61 and measuring the amplitude of the test signal at the antennain step S71.

In step S82 the measured amplitude is compared to the min_amplitude. Incase the measured amplitude is smaller than the min_amplitude, themeasured amplitude is recorded as min_amplitude and the index of thecorresponding phase setting is recorded as min_index in step S8 b.Subsequently, steps S51, S61, S71 and S82 are repeated until themeasured amplitude fulfills the predefined condition which in this caseis the minimum amplitude as defined in FIG. 7.

As soon as the comparison in step S82 reveals that the measuredamplitude is bigger than the min_amplitude, it is checked whether allavailable phase settings have been tested in step S10. Should this proveyes, the method proceeds with step S92 during which the min_index isstored in non-volatile memory.

In the opposite case, steps S51, S61, S71 and S82 are repeated until allpossible values for the phase difference have been run through.

In the end, the min_index indicates the value of the phase differencewhich leads to a difference in phase of 180° between the active loadmodulation signal and the reference signal.

FIG. 13 shows a fifth embodiment of a method according to the presentdisclosure. This embodiment is an alternative realization of theembodiment as described under FIG. 12 which determines the minimumamplitude of the test signal.

Steps S11, S21, S2 a, S31, S41, S4 b, S51, S61, S71 and S8 b comply withthe steps bearing the same numbers described in FIG. 12.

In step S82, the measured amplitude is compared with the min_amplitude.As long as the measured amplitude is smaller than the min_amplitude, themeasured amplitude is recorded as min_amplitude and the correspondingphase setting index is recorded as min_index in step S8 b and the stepsS51, S61, S71 and S82 are repeated.

In the case that the measured amplitude is bigger than themin_amplitude, the phase difference value is modified in step S51 a byconfiguring a predecessor phase setting which realizes a decrease in thephase difference. In step S61 the active load modulation signal isgenerated. The amplitude of the test signal is measured in step S71 andstep S82 is subsequently performed. This second loop starting with S51 ais repeated until the measured amplitude is bigger than themin_amplitude according to the comparison in step S82.

Consequently, when this condition is met, the min_index is stored innon-volatile memory in step S92.

FIG. 14 shows a sixth embodiment method according to the presentdisclosure. In this embodiment, the method is configured to determine anamplitude of the test signal which satisfies the condition of beingequal to the reference amplitude which signifies a 90° phase shiftbetween the active load modulation signal and the reference signal. Thiscondition corresponds to condition C3 as described in FIG. 7.

The present embodiment also has steps S11 and S21 as described in theabove embodiments, for example in FIG. 10.

After reception of the phase calibration command in step S21, theamplitude of the test signal at the antenna is measured in step S00. Asthe active load modulation signal is not yet emitted, the measuredamplitude corresponds to the amplitude of the reference signal whichrepresents the reference amplitude. In step S01, this amplitude isrecorded as ref_amplitude.

The method proceeds with step S2 a in which the initial phase setting isconfigured. In step S31, the active load modulation signal is generated.

In step S41, the amplitude of the test signal on the antenna of the tagdevice is measured.

In step S83, the measured amplitude is compared with the referenceamplitude ref_amplitude. If the measured amplitude is bigger than theref_amplitude, the phase difference is modified in step S51 byconfiguring the next phase setting which leads to an increase of thephase difference. Subsequently, the active load modulation signal isgenerated in step S61 and the resulting amplitude of the test signal ismeasured on the tag's antenna in step S71 during the active loadmodulation signal emission. In step S83 a the measured amplitude iscompared to the ref_amplitude. As long as the measured amplitude isbigger than the ref_amplitude, the steps S51, S61, S71 and S83 a arerepeated as depicted in FIG. 14. If the measured amplitude is smallerthan the ref_amplitude, the loop is exited and the previous phasesetting is stored in non-volatile memory in step S93.

If the comparison in step S83 reveals that the measured amplitude isbigger than the reference amplitude ref_amplitude, the phase differenceis modified in step S51 a by configuring a predecessor phase settingwhich decreases the phase difference. Subsequently, the active loadmodulation signal is generated in step S61 and the amplitude of theresulting test signal is measured in step S71. In step S84 the measuredamplitude is compared to the reference amplitude ref_amplitude. As longas the measured amplitude is smaller than the ref_amplitude, steps S51a, S61, S71 and S84 are repeated in a loop.

If the comparison in step S84 reveals that the measured amplitude isbigger than the reference amplitude ref_amplitude, step S93 is performedand the previous phase setting is stored.

FIG. 15 shows a seventh embodiment method according to the presentdisclosure. In this embodiment, the method is configured to determinethe amplitude of the test signal which corresponds to a 270° phase shiftbetween the active load modulation signal and the reference signal. Forthis to be realized, the amplitude of the test signal shall correspondto the reference amplitude. This embodiment therefore implementscondition C4 as described in FIG. 7.

Steps S11, S21, S00, S01, S2 a, S31 and S41 correspond to the steps withthe same numbers described under FIG. 14. However, for detection of the270° phase shift point C4 as of FIG. 7, in contrast to the 90° phaseshift point C3, the present embodiment first runs through step S84. Thismeans that in step S84 it is tested if the measured amplitude is smallerthan the ref_amplitude. Should this hold true, steps S51, S61, S71 andS83 a are run through as depicted in the left-hand branch of FIG. 15.That means that the phase difference is stepwise increased until anamplitude is measured in the test signal that is bigger than theref_amplitude. Subsequently, the previous phase setting is stored innon-volatile memory in step S93.

In the case in which in step S84 the measured amplitude is bigger thatthe ref_amplitude, steps S51 a, S61, S71 and S84 a are repeated in aloop, until an amplitude is measured which corresponds to theref_amplitude which marks the exit condition. In the loop, according tostep S51 a, the phase difference is stepwise modified by decreasing thephase difference. Each time, an active load modulation signal isgenerated in step S61 using the configured phase setting of step S51 aand the amplitude of the resulting test signal is measured in step S71.The measured amplitude is compared to the ref_amplitude. Should themeasured be bigger than the ref_amplitude, the loop is exited and theprevious phase setting is stored in non-volatile memory according tostep S93.

In the description of FIGS. 10 to 15 the term “next phase setting” andthe term “predecessor setting” refer to the order of values of phasedifferences which are stored in the frontend circuit. It is furthermorepresumed that the values of the phase difference are stored in such anorder that a monotonic increase in phase difference is realized from thefirst to the last stored value. This means that by choosing a next phasesetting, the phase difference is increased, whereas by choosing apredecessor phase setting, the phase difference is decreased.

In contrast to this, the term “previous” represents a time relationshipand refers to a previously performed measurement which is concludedprior to the actual measurement.

The described NFC tag may be used in an implementation of a contactlesscard device realizing, for example, a secure payment application.

The described method and frontend circuit are not limited to the ISO144443 standard but can be applied to any NFC system which uses anamplitude load modulation and therefore requires exact calibration ofthe phase difference. Examples of such NFC systems are described instandards like Felica or ISO 15693.

The described method can alternatively be implemented using a dichotomicsearch algorithm or any other search algorithm as known in the art fordetermining the measured amplitude that fulfills the predefinedcondition. The dichotomic search is a divide and conquer algorithm whichoperates by selecting between two distinct alternatives, nameddichotomies. This implementation allows speeding up the method evenmore. Instead of trying all values of phase differences to find theoptimized setting, the values are divides into two equal groups and ateach iteration of the algorithm, one group is discarded.

For instance when searching for an amplitude of the test signal whichmatches the amplitude of the reference signal in the cosine functiondepicted in FIG. 7, the algorithm e.g. tries two settings spaced by 180degrees, for example at 210 and 30 degrees. 30 degrees will result in anamplitude of the test signal above the reference and the other one willresult in an amplitude below the reference. Next, the setting in betweenis tried, i.e. 300 degrees. If for this setting the measured amplitudeis above the reference Vref, then the settings within the range between300 and 30 degrees are discarded and the search algorithm iterates onthe range between 210 and 300 degrees.

What is claimed is:
 1. A method of phase calibration in a frontend circuit of a near field communication (NFC) tag device, the method comprising: receiving a reference signal generated by an NFC signal generator device; generating an active load modulation signal with a preconfigured value of a phase difference with respect to the reference signal of the NFC signal generator device; measuring an amplitude of a test signal present at an antenna of the NFC tag device, the test signal resulting from overlaying the reference signal with the active load modulation signal; repeating steps (a)-(d) until the measured amplitude fulfills a predefined condition: (a) modifying the value of the phase difference; (b) providing the active load modulation signal with the modified value of the phase difference; (c) measuring an amplitude of the test signal; and (d) comparing the measured amplitude with the previously measured amplitude or with a reference amplitude; and storing the value of the phase difference corresponding to the previously measured amplitude.
 2. The method according to claim 1, wherein the predefined condition comprises: the measured amplitude is smaller than the previously measured amplitude; the measured amplitude is bigger than the previously measured amplitude; the measured amplitude is smaller than the reference amplitude; or the measured amplitude is bigger than the reference amplitude.
 3. The method according to claim 1, further comprising: measuring an amplitude of the test signal during non-emission of the active load modulation signal; and recording this amplitude as the reference amplitude.
 4. The method according to claim 1, wherein the active load modulation signal is generated in function of an internal carrier signal and a data signal.
 5. The method according to claim 4, wherein the internal carrier signal is generated using the reference signal and wherein a frequency of the internal carrier signal is adapted to a frequency of the reference signal.
 6. The method according to claim 1, wherein the modifying the value of the phase difference realizes an increase or a decrease of the phase difference with respect to the reference signal.
 7. The method according to claim 1, wherein the values of the phase difference are retrieved from a memory in the frontend circuit or in a host component of the NFC tag device.
 8. The method according to claim 1, further comprising receiving a phase calibration command generated by a host component of the NFC tag device.
 9. The method according to claim 1, wherein the test signal comprises voltage values which are proportional to a voltage at the antenna.
 10. The method according to claim 1, wherein between two consecutive amplitude measurements of the test signal a period during which the active load modulation signal is not emitted is present.
 11. A control unit for use in a frontend circuit of a near field communication (NFC) tag device, the control unit comprising: an input configured to receive a reference signal; an output configured to provide phase related information; and control circuitry configured to: generate an active load modulation signal with a preconfigured value of a phase difference with respect to the reference signal; measure an amplitude of a test signal that results from overlaying the reference signal with the active load modulation signal; repeat steps (a)-(d) until the measured amplitude fulfills a predefined condition: (a) modify the value of the phase difference; (b) provide the active load modulation signal with the modified value of the phase difference; (c) measure an amplitude of the test signal; and (d) compare the measured amplitude with the previously measured amplitude or with a reference amplitude.
 12. The control unit according to claim ii, wherein the predefined condition comprises: the measured amplitude is smaller than the previously measured amplitude; the measured amplitude is bigger than the previously measured amplitude; the measured amplitude is smaller than the reference amplitude; or the measured amplitude is bigger than the reference amplitude.
 13. The control unit according to claim ii, wherein the control circuitry is further configured to measure an amplitude of the test signal during non-emission of the active load modulation signal, and to record the measured amplitude as the reference amplitude.
 14. The control unit according to claim ii, wherein the active load modulation signal is generated as a function of an internal carrier signal and a data signal.
 15. The control unit according to claim ii, wherein the modified phase difference is higher or lower than the phase difference with respect to the reference signal.
 16. A front end circuit for a near field communication (NFC) tag comprising: a test signal input configured to receive a test signal and a reference signal, wherein the test signal is proportional to a signal occurring at an antenna that can be connected to the front end circuit and wherein the reference signal is generated by an NFC signal generator device a signal output for providing an active load modulation signal a control unit which is configured to run a phase calibration process and to provide a value of a phase difference and a data signal; a receiver circuit coupled to the test signal input and configured to perform an envelope detection on the reference signal and the test signal; a measurement circuit coupled to the receiver circuit and to the control unit, the measurement circuit being configured to provide an amplitude of the test signal; a clock generator circuit which is coupled to the receiver circuit and to the control unit, the clock generator circuit being adapted to generate an internal carrier signal using the value of the phase difference; and a driver circuit coupled to the clock generator circuit and to the signal output, the driver circuit being configured to provide the active load modulation signal as a function of the internal carrier signal and the data signal; wherein the phase calibration process comprises: receiving the reference signal; generating the active load modulation signal with a preconfigured value of a phase difference with respect to the reference signal of the NFC signal generator device; measuring an amplitude of the test signal, the test signal resulting from overlaying the reference signal with the active load modulation signal; repeating steps (a)-(d) until the measured amplitude fulfills a predefined condition: (a) modifying the value of the phase difference; (b) providing the active load modulation signal with the modified value of the phase difference; (c) measuring an amplitude of the test signal; and (d) comparing the measured amplitude with the previously measured amplitude or with a reference amplitude; and storing the value of the phase difference corresponding to the previously measured amplitude.
 17. The front end circuit according to claim 16, wherein the measurement circuit comprises an analog-to-digital converter circuit.
 18. An NFC tag device comprising: a frontend circuit according to claim 16; an antenna; a host component coupled to the frontend circuit through a digital interface; and an impedance matching circuit coupled between the frontend circuit and the antenna.
 19. The NFC tag device according to claim 18, wherein the impedance matching circuit comprises a series capacitor coupled between the antenna and the test signal input of the frontend circuit.
 20. The NFC tag device according to claim 19, wherein the series capacitor forms a capacitive divider with a capacitance of the test signal input. 